1. Field of the Invention
The present invention relates to a transparent conductive thin film, process for producing the same, sintered target for producing the same, and transparent, electroconductive substrate for display panels and organic electroluminescence devices, more particularly a transparent conductive thin film which can be produced easily by sputtering or the like with a sintered target, needs no post-treatment such as etching or grinding, is low in resistance and excellent in surface smoothness, and has a high transmittance in the low-wavelength region of visible rays; and transparent, electroconductive substrate for display panel and organic electroluminescence device excellent in light-emitting characteristics, both including the transparent conductive thin film.
2. Description of the Prior Art
A transparent conductive thin film has a high electroconductivity and high transmittance in the visible light region. Therefore, it has been widely used as a transparent electrode for various devices, e.g., solar cells, liquid-crystal displays (LCDs), display panels with an electroluminescence device, and various types of light-receiving devices.
It has been also used for heat ray reflecting films for windows of automobiles, buildings or the like, various types of antistatic films, and transparent heating devices for anti-fogging purposes for showcases containing frozen foods.
An electroluminescence device (hereinafter referred to as EL device) is a device that utilizes electroluminescence, and has been attracting much attention as a light-emitting device for various types of displays, because of its high visibility and resistance to impact, the former coming from its self-luminescence and the latter from being completely solid. EL devices fall into two general categories of inorganic and organic devices, the former using an inorganic compound as the light-emitting material and the latter an organic compound. Organic EL devices have been extensively studied for commercialization of displays of the next generation, because they can be easily made compact by greatly reducing the driving voltage. An organic EL device is composed of a laminate of anode/light-emitting layer/cathode as the basic structure with a transparent conductive thin film as the anode formed on a transparent, electrically insulating substrate, e.g., glass sheet, where light is normally emitted from the substrate side.
Various materials have been extensively used for transparent conductive thin films. They include tin oxide (SnO2) doped with antimony or fluorine, zinc oxide (ZnO) doped with aluminum or gallium, and indium oxide (In2O3) doped with tin. In particular, films of indium oxide (In2O3) doped with tin, or In2O3—Sn-based films referred to as ITO (indium tin oxide) films, have been widely used because of easiness of producing low-resistance films.
The known processes for producing thin ITO films include spraying, vacuum evaporation, sputtering and ion plating. Sputtering is an effective process for forming a film of a compound of low vapor pressure on an object (hereinafter merely referred to as substrate), or forming a film whose thickness should be precisely controlled. It has been widely used because it can be handled by a simple procedure. Ion plating is a process that ionizes the particles evaporated in a manner similar to that for sputtering to improve their adhesive strength to the substrate, and then accelerates them in an electrical field onto the substrate, on which the film is securely formed.
Of these processes, sputtering is a film-making process which generates an argon plasma by a glow discharge produced between the substrate as the anode and a target as the cathode in an inert atmosphere (argon) kept at 0.1 to 10 Pa, and directs the resulting argon cations onto the target as the cathode to scatter the target component particles out of the cathode, which are deposited on the substrate.
This process is classified by method for producing the argon plasma; radio-frequency (RF) sputtering when radio-frequency (RF) plasma is used, and DC sputtering when DC plasma is used. The film can be produced by focusing argon plasma on immediately above the target by a magnet provided on the backside of the target. This method is magnetron sputtering, which can have high argon ion collision efficiency even at low gas pressure. DC magnetron sputtering is normally adopted to produce transparent conductive thin films of ITO.
Sintered ITO is normally used as the target. It is produced by a powder sintering process, in which indium oxide or tin oxide is incorporated to have a substantially intended composition, pressed and sintered at 1400° C. or higher.
The target is normally of sintered ITO containing tin oxide (SnO2) at around 10% by weight, in particular the one having a density below 7.0 g/cm3. More recently, sintered ITO of higher density and the target using such a sinter are being developed to improve film-making characteristics of ITO.
A process for producing sintered ITO of high density (density: 7.02 g/cm3 or more, or 98% or more as relative density) and high uniformity (scattering of around 1%) is disclosed by Japanese Patent Laid-open Publication No.2000-144393. Sputtering with this ITO target gives good films during the initial stage, but deteriorates in sputtering performance (sputter rate) as the process time nears the final stage, because a black substance called nodule is generated on the target surface to cause problems, e.g., abnormal discharge. This phenomenon results from uncontrolled pore distribution in the sinter. This means that its effects cannot be negligible, when sputtering lasts a long time.
The electrode for LCDs and organic EL devices needs a transparent conductive thin film of surface smoothness and low resistance. In particular, an electrode for displays with an organic EL device needs a transparent conductive thin film of a highly smooth surface, because a superthin film of organic compound is formed thereon. Surface smoothness generally depends greatly on crystallinity of the film, and an amorphous film free of grain boundaries has a better surface smoothness than the others of the same composition.
Even the ITO film of conventional composition is amorphous and has good surface smoothness, when formed by evaporation on a substrate kept at a low temperature (e.g., to 150° C. or lower) during the film-making process. However, the film prepared by evaporation is insufficient in density and adhesion to the substrate, and it is also insufficient in film stability and repeatability. Therefore, it is not suitable for mass production of transparent conductive thin films. On the other hand, sputtering is expected to give a film of excellent surface smoothness, when effected without heating the substrate during the film-making process and at a high sputtering gas pressure (e.g., 2 Pa or more), because the film tends to be amorphous.
However, resistivity of the amorphous ITO film thus prepared is limited to 6×10−4 to 8×10−4 Ω·cm. Therefore, it should have a sufficient thickness to form an electrode of low surface resistance to be useful for displays e.g., LCDs and organic EL devices. However, the ITO film may have a problem of coloration when its thickness exceeds 500 nm.
The ITO film, even when prepared by sputtering without heating the substrate, may be locally heated, because of high kinetic energy of the sputtered grains incident on the substrate, to form the mixed film of the amorphous phase contaminated with the fine crystalline phase. This tendency is more noted as sputtering gas pressure decreases.
The fine crystalline phase present in the ITO film can be detected by a transmission electron microscope, in addition to X-ray diffractometer. It can deteriorate surface smoothness of the film, even when produced only locally. Moreover, it may cause problems related to post-treatment, because it may selectively remain unremoved by etching with a weak acid, which is a necessary post-treatment step to remove fine projections on the surface for surface smoothness.
Some processes for stably producing completely amorphous ITO films have been proposed. For example, Japanese Patent Laid-open Publication No. 4-48516 discloses a process for producing an amorphous ITO film by sputtering the target while keeping the substrate at 100 to 120° C., which is lower than crystallization temperature of ITO (around 150° C.). Japanese Patent Laid-open Publication No. 3-64450 discloses a film-making process with hydrogen gas introduced into the oxygen-containing inert gas for sputtering.
However, the ITO film prepared at low temperature, although easily patterned by wet etching because it is amorphous, involves problems of increased electrical resistivity and decreased visible rays transmittance. The process that includes photolithography for etching the amorphous ITO film formed on a substrate with hydrogen gas introduced in the sputtering step and annealing for crystallization of the film cannot form the amorphous film at a sufficiently high film-making rate. The complicated process is another disadvantage.
Japanese Patent Laid-open Publication No. 62-202415 discloses a process for producing an indium oxide film doped with silicon or silicon and tin by RF sputtering or electron beam evaporation. This process can produce an indium oxide film doped with silicon or silicon and tin free of film defects, but the publication is silent on crystalline structure of the film, from which it is considered that it cannot give a film of surface smoothness even when it is prepared by RF sputtering in a pure argon gas atmosphere.
A Si-doped indium oxide film is totally crystalline, as observed by X-ray diffractometry, so it is not excepted to be an amorphous film that is excellent in surface smoothness (refer to Appl. Phys. Let., vol. 64, 1994, p. 1395).
An In2O3—ZnO-based film is known as a transparent, electroconductive film that is amorphous and excellent in surface smoothness, as disclosed by, e.g., Japanese Patent Laid-open Publication No. 6-234521. This film can keep its properties even when exposed to heat of 200° C., but it has a lower transmittance than that of an ITO film in the low wavelength region of visible rays, because it contains metallic Zn. However, it may have the problem of unstable properties, because it is known that metallic Zn or ZnO present in the film tends to react with carbon dioxide and moisture in air. Therefore, an In2O3—ZnO-based film is functionally insufficient for an LCD or organic EL device electrode.
Japanese Patent Laid-open Publication No. 11-323531 discloses a transparent conductive thin film of an amorphous In—Ge-based material. However, it has a high electrical resistivity of 8×10−4 Ω·cm or more, and is unsuitable for an LCD or organic EL device electrode.
More recently, an attempt has been made to use a transparent cathode for an EL device and to emit light from the cathode side. A light-receiving device transparent as a whole can be made when both anode and cathode are made transparent. The transparent light-emitting device can perform colorful display while it is not emitting light, when an optional color is used as a background color, to improve its decorative characteristics. For example, when black color is used as the background color, the device can have improved contrast while it is emitting light. The EL device can be provided with a color filter or color conversion layer as an accessory without considering the accessory while it is produced, because the accessory can be placed on the light-emitting device. This provides the advantage of reducing electrode resistance, because it can be produced while increasing substrate temperature.
Under these situations, an attempt has been made recently to produce an organic EL device with a transparent cathode. For example, Japanese Patent Laid-open Publication No. 10-162959 discloses an organic EL device having a structure with an electron-injecting metallic layer coming into contact with the organic layer by placing the organic layer containing an organic light-emitting layer between the anode and cathode, the latter being composed of the electron-injecting metallic layer and amorphous, transparent, electroconductive layer. Japanese Patent Laid-open Publication No. 2001-43980 discloses an organic EL device with a transparent cathode and light-reflecting metallic layer, e.g., that of Cr, Mo, W, Ta or Nb, for the anode so that it can emit light more efficiently from the cathode.
However, each of these EL devices, containing an In2O3—ZnO-based film as the transparent conductive thin film, has the problems of lower transmittance than an ITO film in the low wavelength region of visible rays, as discussed earlier, and unstable characteristics.
A transparent conductive thin film very high in surface smoothness, low in resistance and high in transmittance is an essential part for various display panels, e.g., LCDs and organic EL devices, which are recently becoming increasingly more precise and fine. Therefore, there is a large demand for easy processes capable of producing a transparent conductive thin film high in surface smoothness, low in resistance and high in transmittance.